1. Field of the Invention
The present invention relates to a process for recovering 3,4-epoxy-1-butene (EpB) and particularly, to a process for recovering EpB from a vapor phase catalytic oxidation reactor effluent where 1,3-butadiene is reacted with oxygen over a silver catalyst. More specifically, the present invention pertains to a method for the recovery of EpB wherein the EpB laden reactor effluent gas is contacted with a counter-currently moving absorbent stream at temperature of from about 0.degree. C. to about 40.degree. C. and pressures less than about 4 bar absolute.
2. Description of the Prior Art
3,4-Epoxy-1-butene also known as butadiene monoxide and vinyl-oxirane, is an important compound and generally has uses as an intermediate for preparing materials such as tetrahydrofuran and 1,2-butylene oxide. Methods for preparing EpB are disclosed in U.S. Pat. Nos. 5,618,954, 5,362,890 and 4,897,498, the disclosures of which are incorporated herein by reference. One method for the manufacture of EpB so described generally includes the selective epoxidation of 1,3-butadiene (referred to herein as butadiene). The butadiene is contacted with an oxygen-containing gas in the presence of certain silver catalysts.
The rate of epoxidation in the reactor for a given total pressure is related to the mole fraction of oxygen in the epoxidation reactor feed gas. U.S. Pat. No. 5,362,890 discloses that high oxygen concentrations enhance the reaction of 1 ,3-butadiene to 3,4-epoxy-1-butene. Therefore, it is highly advantageous in the production of 3,4-epoxy-1-butene from 1,3-butadiene to operate the epoxidation reactor with a feed gas containing as high an oxygen content as possible, concomitant with safe operation outside of the explosive limits.
As explained in Lees, F. P., "Loss Prevention in the Process Industries, Volume 1," 485-86 (1980), a flammable gas, e.g., methane, butane, and other alkane hydrocarbons, burns in oxidizing environments only over a limited composition range. The limits of flammability (often called the explosive limits) are the concentration extremes of: high oxygen and low combustible mixture, and low oxygen and high combustible mixture at which a mixture of a flammable gas and an oxidant can continue to burn once a flame is ignited by an external energy source such as a spark. These flammability extremes are a function of temperature, pressure, and composition. The explosive limit is usually expressed as volume or mole percent flammable gas in a mixture of oxidant (usually oxygen), inert, and flammable gas. The smaller value is the lower (lean) limit and the larger value is the upper (rich) limit. Typically, the safe operating range for a given flammable gas and oxidant mixture decreases as temperature and pressure increase, and amount of inert decreases. Increases in pressure have a larger effect than the increases in temperature. The explosive limits of flammable gas mixtures, e.g., n-butane and 1,3-butadiene, can be estimated by the well-known LeChatlier's rule.
EpB is a very reactive compound which requires the recovery operations to be performed under mild conditions to avoid the conversion of EpB to other undesired compounds such as butenediols and oligomers. It is possible to recover EpB directly from the epoxidation effluent by compressing the gaseous effluent to pressures sufficiently high to liquefy the EpB. However, the compression of the effluent would require the use of a series of compressors and heat exchangers to remove the heat of compression and maintain the EpB at a temperature which would minimize by-product formation.
The recovery of gaseous products by absorption wherein a gaseous stream is contacted with a liquid absorbent, also referred to as an extractant or solvent, is well known. For example, in ethylene oxide processes wherein ethylene is epoxidized to ethylene oxide, ethylene oxide is recovered from the epoxidation reactor effluent gas by counter-current contact with a solvent having water as the main component. Crude ethylene oxide is recovered from the EO-laden water stream from the bottom of the absorption zone by distillation or depressurization and stripping with steam. The water remaining after stripping of ethylene oxide is recycled to the EO absorption zone for reuse. Such a system is described, for example, by Dever et al in the Kirk-Othmer Encyclopedia of Technology, 4.sup.th Edition, "Ethylene Oxide", 930-933 (1994).
One problem with the above-described recovery process is a significant amount of the ethylene oxide reacts with the water to produce ethylene glycol. During the recovery of ethylene oxide vapor in the absorption column and during the subsequent stripping or distillation of the recovered ethylene oxide-water mixture, it is impossible to prevent the conversion of a substantial fraction of the ethylene oxide to ethylene glycol via the reaction with water. Such losses to ethylene glycol can reach 3 to 20 percent or higher of the ethylene oxide originally present in the gas phase reaction effluent and represent a large economic penalty for operations where ethylene glycol is not a desired product. However, since EpB has very limited water solubility, water is not a practical absorbent for the recovery of EpB. Accordingly, other methods of separating the EpB from the other epoxidation reactor effluent constituents have been desired.
U.S. Pat. No. 5,117,012 discloses a process for the recovery of EpB by contacting the vaporous oxidation reactor effluent having EpB, butadiene, an inert diluent gas, and oxygen with liquid butadiene in an absorption zone to obtain a solution of EpB in butadiene. The inert diluent gases specifically contemplated by the '012 patent are nitrogen and methane. The '012 patent discloses contacting the reactor effluent with liquid butadiene at a pressure of about 5 to 15 bar and at a temperature of about 0.degree. C. to about 30.degree. C. Such high pressures result in a number of disadvantages, such as, the capital and operating costs for the compressor(s) required to achieve the high pressures, EpB losses through hydrolysis and oligomer formation caused by the temperatures produced by the compression of the reactor gas effluent, and butadiene losses due to polymerization resulting in lower overall yields and downtime.
U.S. Pat. No. 5,312,931 discloses a process for the recovery of 3,4-epoxy-1-butene from a vapor phase epoxidation effluent by counter-current contact with a mixture of liquid butane and butadiene in an absorption zone. The absorber is operated at a pressure of from about 3 bars to about 6 bars and a temperature of about 0.degree. C. to about 40.degree. C. Although the '931 patent teaches an improvement over the '012 patent, the butane/butadiene recovery process also has disadvantages. In order to ensure that the absorbent butane/butadiene largely remains a liquid within the absorption zone at operating temperatures that can be achieved with an inexpensive cooling medium such as water, i.e., above at least about 30.degree. C., the absorption zone must be operated at a pressure of at least about 4.2 bars. Because of the dominance of pressure effects on explosive limits, the maximum safe oxygen content of the epoxidation recycle gas is generally dictated by the highest pressure point in the recycle loop, i.e., by the pressure at the outlet of the recycle compressor. For example, with a typical mixture of butadiene and n-butane diluent in the reactor effluent gas, the maximum safe oxygen content at a compressor outlet pressure of about 5 bar or greater and a reasonable compressor outlet temperature of 85.degree. C., is less than 28 mole percent.
If the absorption zone could be operated at lower pressure and the compressor outlet pressure could be lowered, e.g., to less than 4 bar, then the maximum safe oxygen content would be greater than about 31 mole percent oxygen. For a given oxygen concentration, the process could be operated farther from the dangerous upper explosive limit and would be safer. However, operating at lower pressures, and concomitantly lower temperatures is quite costly if the required low temperature cooling is supplied by ordinary means to those skilled in the art such as chilled brine or glycol refrigeration units. Moreover, high inlet reactor pressure, i.e., above about 3 bar, adversely affects EpB production. The reactor effluent normally exits the reactor at low pressure, e.g., less than 3 bar, normally 1.0 to 2.0 bar. To meet the aforementioned temperature and pressure requirements for absorption with a mixture of butane and butadiene, the reactor effluent is first compressed to a suitable pressure, i.e., greater than about 4.2 bars, prior to its introduction into the absorption zone. The higher pressures and resulting polytropic temperature rise within the compression zone in the presence of high concentrations of 3,4-epoxy-1-butene can cause formation of polymeric materials which deposit on the walls of the compressor and associated piping. The build-up of such polymeric material reduces the operating efficiency of the compressor and can lead to permanent equipment damage and frequent process shutdowns for maintenance, with subsequent loss of production and revenues. Moreover, the compression ratio of the compressor would be reduced, resulting in lower equipment cost.
Accordingly, there is a need for a process for efficiently and economically recovering 3,4-epoxy-1-butene from a vapor phase epoxidation reactor using low temperature and pressure conditions.